This invention relates to a color picture synthesizer, and more particularly to a synthesizer wherein chroma-key is used.
Heretofore, there have been two types of color picture synthesis. The one system is analog signal synthesis and the other system is digital signal synthesis. As shown in FIG. 3, a high speed switching signal which is delivered from a data controller 301 controls a superimpose controller 302 for connecting either one of the two input analog color picture as the output.
FIG. 4 shows an example wherein a chroma-key is used. A chroma-key extraction circuit 402 extracts a specified color signal from an analog color picture 2, and this extracted key signal deletes the analog color picture 2 in an analog synthesizer 401. Color picture synthesizers with chroma-key are disclosed in a Japanese patent application entitled "A chroma-key apparatus" and laid open as a Provisional Publication No. 211791/90 and another Japanese patent application entitled "A magnetic recording and reproducing apparatus with a built-in camera" and laid open as a Provisional Publication No. 318385/89.
FIG. 5 shows a digital synthesizer where two digital color pictures are synthesized in a digital synthesizer 501. A prior art of digital synthesis is disclosed in a Japanese patent application entitled "Digital superimpose system" and laid open as a Provisional Publication No. 92170/90.
In the prior arts shown in FIG. 3 and FIG. 4, an analog switch (not shown in the drawings) operated by the switching signal or by the key signal has an inevitable time lag in operation. The time lag causes a slippage or a blur at a boundary of switching. And in a chroma-key synthesis, insufficient color extraction also causes a blur. In a case when a chroma-key is used, for example, to take out a designated color component from digitized signals of an analog color picture, and to fill up the space with another digital color picture, the prior art disclosed in the "Digital superimpose system" of the Provisional Publication No. 92170/90, can not be used, because there are intensity fluctuations of the designated color component.
In the prior art entitled "A chroma-key apparatus", the intensity level of the designated color component is used for determining stretching and clipping levels of a color selection circuit. This process is efficient to optimize the chroma-key level, but the process is insufficient in the accuracy of optimization.
In recent years, window systems for displaying other pictures such as pictures reproduced from video tapes in a display of a color picture produced in a computer, have been developed. In such window systems, there is a wide variety of analog color signal intensities among individual products of a same apparatus. A color picture synthesizer of the prior art can not cope with these intensity fluctuations.